The invention relates to a process for joining components made of metal, in particular aluminum and aluminum alloys, by means of a thermally-curable (hardenable) adhesive deposited between the close-fitting surfaces of the components that are to be joined, wherein one of the outer surfaces neighboring the said close-fitting surface of one of the components is radiated with infra-red radiation to cure the adhesive.
Among the methods used to join metallic components adhesive bonding is gaining increasingly in importance. Also, in the automobile industry increasing efforts are being made to replace or supplement spot welding by adhesive bonding. An example of this is the joining of the inner and outer sheets of the bodywork. The strength requirements made of bodywork joints can be satisfied reliably today only by means of thermally-curable adhesives. In practice therefore the problem arises as to how the heat for curing the adhesive can be optimally applied. The following possibilities are available for that purpose:
The whole structure can be placed in a furnace. For large parts this means there must be a large furnace of sufficient capacity. One disadvantage of this process is that the whole structure must be heated up. As a result other parts could be damaged by the heat e.g. electrical wising or thermoplastic fittings.
The parts bearing adhesive are treated with hot air. With this method there is the problem that, especially where there are metals involved that conduct heat readily, for example components of aluminum or aluminum alloys, heat is lost rapidly by thermal conduction. In order to maintain a high temperature locally during the curing operation, it is necessary, therefore, to supply a correspondingly large amount of heat.
It is also possible to achieve localized heating by means of direct contact with flexible thermal cushions and bindings. This method, however, requires good contact and therefore careful attachment of the heating elements, which in turn requires access to the joint from all sides.
Using infra-red radiation a large amount of heat can be applied, easily and locally, to compensate for heat losses due to thermal radiation from metals, in particular aluminum and aluminum alloys. The high reflectivity of metallic surfaces, however, reduces the absorbtion of heat and can lead to the infra-red sources becoming overheated and to soldered or brazed joints melting.
A means of overcoming the above mentioned disadvantages of curing with infra-red radiation is known from EP-B-O 311 323 in that a coating that absorbs infra-red radiation is applied to the surface to be radiated i.e. the outer surface neighboring the close-fitting surface of one of the components to be joined. The coating, normally black acrylic resin, is applied prior to radiation only to part of the outer surface of the components to be joined. After the curing operation, the black resin coating has to be removed using solvents.